Fiber optic vibration sensors have been used in a wide variety of applications, from monitoring machinery conditions to detecting motion for alarm systems. When light passing though a fiber optic sensor passes through a bend in the fiber, that light is susceptible to frequency change due, to vibration. This has led to the development of fiber optic sensors where an optical fiber is bent, cut at the apex of the bend, and a grid element is interposed at the bend. FIG. 1 shows an embodiment of a fiber optic vibration sensor. The grid element 2 is the terminus of a flange 4, and is referred to as a sensor reed. As the reed vibrates, the grid, which has numerous slits, disrupts the light crossing the cut 6 in the optical fiber 8, thereby changing the light frequency passing through the optical fiber. Measurements of the changes in the light frequency can therefore be used as a sensitive and accurate vibration detector. In practice, the sensor reed is mounted in a small box-like structure 10 referred to as a sensor case.
The sensor reed will itself have a base frequency, meaning that its vibration is a known factor used in the calculations of the changes in light frequency. The base of the sensor reed, however, is affected by physical stresses on the sensor reed, such as how the sensor reed is mounted inside of the sensor case 12. Therefore, the initial mounting of the sensor reed is a delicate procedure, and has thus far been a permanent, non-adjustable mounting. In many cases the frequency of the sensor reed is adjusted by inserting epoxy or machining the sensor reed to fix the base frequency.
Problems, however, arise, in that the sensor reed is susceptible to frequency drift over time, which is when the base frequency changes to an unknown, or undesirable frequency. Currently, the only solution to this is to, completely replace the whole sensor case, which includes replacing the optical fibers attached to the sensor case, as well as other integrated parts. Similarly, if it is ever desired to operate the sensor reed at a different frequency, it cannot be adjusted and the sensor case needs to be replaced. This is an expensive and time consuming effort. What is needed is a fiber optic sensor that has a sensor reed with an tuneable base frequency.